1. Field of the Invention
The invention relates to a clamping structure for a solid oxide fuel cell stack. More particularly, the invention relates to a planar solid oxide fuel cell stack which is compressed using a clamping structure which includes a four-sided planar flexible sheet.
2. Description of the Related Art
A planar solid oxide fuel cell (SOFC) stack consists of a repeated sequence of solid oxide fuel cells across which an electrical voltage is created alternating with interconnects.
The stack typically includes 5 to 200 fuel cells and consists of a sequence of fuel cells comprising an anode, a cathode and a solid oxide electrolyte each fuel cell alternating with the interconnect. The fuel cells are provided with fuel and oxidant by a manifold system via an internal channel system. Fuel and oxidant are distributed from layer to layer in the fuel cell stack by a channel system. During operation, an electrochemical voltage is created across the individual fuel cells. The interconnect serves to introduce oxidant and fuel to the fuel cells in separate channels and to collect electrons from one fuel cell and transmit and deliver them to an adjacent fuel cell.
The walls of the internal channel system must be gas tight in order to avoid leakages of gas to the external surrounding or untimely mixing of oxidant and fuel. This is ensured by using a sealing material of for instance glass, and/or by providing an intimate and direct bonding between the fuel cell and interconnect on the available sealing surfaces.
The gas tight behaviour and the desired electrical contact between the fuel cells and interconnects are ensured in a SOFC stack by pressing the fuel cells and interconnects together with a well-defined compressive force using a clamping structure. In some cases the required compressive force can be as high as 100 N/cm2 across each fuel cell surface during operation of the fuel cell stack. The magnitude of the compressive force is dependent on the actual design of the interconnect and fuel cell and on the gas pressure during operation. The compressive force is provided on the end surfaces of the stack.
A SOFC stack typically operates at temperatures of 600-850° C. Such high temperatures represent a challenge to the design of the mechanical clamping structure required to generate compressive forces of such a magnitude.
It is important that the compressive force is exerted on a surface area corresponding to the surface area of the fuel cells in the stack. The inner sections of the end surfaces of the stack must be compressed in order to maintain electrical contact and the peripheries of the end surfaces must be compressed in order to make the stack gas tight. Conventionally, fuel cells have surface areas of 80-1000 cm2 and compressive forces of up to 100,000 N can be required.
Various types of clamping structures or assemblies are known for instance assemblies using bands for compressing planar fuel cell stacks. U.S. Pat. No. 5,993,987 discloses a fuel cell stack comprising at least one band circumscribing end plates and interposed electrochemical fuel cells. A resilient member cooperating with the band urges the end plates towards each other thereby applying compressive force to the fuel cells to promote sealing and electrical contact between the layers forming the fuel cell stack.
US patent application No. 2006093890 discloses a fuel cell stack maintained in compression by a strap assembly that includes a compressive band extending around the end plates of the fuel cell stack.
Traditional clamping structures are based on the compression of a metallic planar end plate flange placed at either end surface of the SOFC stack and extending beyond the surface area defined by the fuel cells in the stack. The two end plate flanges are connected to each other at their periphery external to the fuel cells by a clamping structure of tie-rods, pipe sections, springs and nuts for creating a compressive force in the stack.
The forces experienced in the tie-rods can be established with the aid of the elasticity of the tie-rods using disc springs, coil springs, gas springs or using pneumatic cylinders or hydraulic cylinders.
SOFC stacks typically operate at temperatures of 600-850° C. At this temperature most metallic materials when subjected to mechanical stress will creep with time. It is therefore advantageous to maintain the metallic sections that experience mechanical stress at as low a temperature as possible.
The tie-rods are typically inserted through the two planar end plate flanges, thereafter through pipe sections of a specified length extending beyond the SOFC stack and through springs placed at the ends of the pipe sections. The pipe sections function as spacers for distancing the springs from the fuel cell stack such that the springs are maintained at a less severe operating temperature than the high temperature experienced during operation of the stack. Nuts positioned after the springs are used to assemble these components and thereby to adjust the compressive force on the SOFC stack.
During operation of the SOFC stack the tie-rods are at a temperature approximately equivalent to the stacks operation temperature. The tension created thereby in the tie-rods results in a tendency of the tie-rods to creep.
During operation of the SOFC stack the planar end plate flanges are also subjected to mechanical tension during influence of the forces from both the tie-rods in the clamping system and the stack causing creep of the planar flanges. The planar flanges therefore tend to become convex in form.
In an alternative clamping structure the tie-rods and the planar end plate flanges are during operation at a much lower temperature than the SOFC stack's operation temperature. This is made possible by thermally insulating the SOFC stack at the sides of the stack using insulation material. Placing additional insulation material at either end of the stack adjacent to the planar end plates allows a transfer of the compressive force obtained during clamping through the additional insulation material. The tie-rods and the planar end plate flanges can thus experience greater tension before undesirable creep sets in. The disadvantage of these types of clamping structures using tie-rods are associated with the planar end plate flange placed at each end surface of the SOFC stack and extending beyond the surface area defined by the fuel cells in the stack. Each planar end plate flange experiences a bending force when exposed to the mechanical forces originating from the tie-rods and the stack.
These undesirable effects result in a reduction of the compressive force on the whole stack or in an uneven distribution of the compressive force on the stack leading to poorer electrical contact and/or the stack becomes less gas tight and leakage of gas to the external surroundings cannot be avoided.
The flanges used are therefore of a sizeable thickness, typically 5-20 mm, in order to absorb these forces and minimise the deformation of the flanges, while simultaneously preventing gas leakage and loss of electrical contact in the stack.
WO patent application No. 2006/012844 discloses a fuel cell stack for solid oxide fuel cells with a clamping device and a heat insulating device. The heat insulating device is located between the fuel cells and the clamping device, which has pressure distribution elements in the form of either flat plates that are parallel to each other, a hemi-spherical shell or are semi-cylindrical. The pressure distribution elements ensure that the pressure is distributed uniformly on the entire surface of the heat insulating elements.
No details are given regarding the construction of the pressure distribution elements, but it is known in the art to use flat plates that are of metal. Furthermore the application of hemi-spherical shells implies the use of a rigid or hard material shaped in the form of a hemi-sphere.
Generally pressure distribution elements in the form of flat plates are manufactured from metal. Pressure distribution shells or cylinders of metal can be prepared by metal forming processes such as deep drawing, which is a more complicated process than the process used in preparing flat plates.
The economy associated with solid oxide fuel cells is high and there is a constant need for a reduction in the cost of SOFC stacks without any losses in the chemical and/or physical properties of the various stack components.
Furthermore, there is also a need for solid oxide fuel cell components that show acceptable physical properties while contributing to a reduction in weight and/or volume of the stack.
It is an objective of the invention to provide a clamping structure for a planar SOFC stack in which deformation due to uneven distribution of compressive forces is avoided during operation of the SOFC stack.
It is a further objective of the invention to provide a planar SOFC stack having reduced weight and volume.